After beating the game, or at least a level, you'll notice that you'll have a total of five unlockable cheats when attempting to replay a level via the "level select" option. Here's how to unlock them.

Unlockable

How to unlock

Bullet Cam on Every Kill

Find every clue.

One-Hit Kill

Beat game on Hardcore with Free Aim.

Unlimited Ammo

Collect all golden gun pieces.

Unlimited BulletTime

Earn a gold-medal score (about 7,000) or better on every level in arcade mode.